The fsck command invokes file-system-specific
programs to check the special devices listed in the
fstab(5) file or on the command
line for consistency.

It is normally used in the script
rc(8) during automatic reboot. If no
file systems are specified, fsck reads the
file fstab(5) to determine which
file systems to check and in what order. Only partitions in fstab that are
mounted “rw” or “ro” and that have non-zero pass
numbers are checked. File systems with pass number 1 (normally just the root
file system) are checked one at a time. When pass 1 completes, all remaining
file systems are checked, with one process spawned per disk drive. The disk
drive containing each file system is inferred from the longest prefix of the
device name that ends in a digit; the remaining characters are assumed to be
the partition designator. By default, file systems which are already mounted
read/write are not checked.

Causes fsck to use the specified block
as the location of the superblock. Block 32 is usually an alternate super
block. This option is only valid for filesystems that support backup
superblocks (ffs and ext2fs).

Limit the number of parallel checks to
maxparallel. By default, the limit is the
number of disks, running one process per disk. If a smaller limit is
given, the disks are checked round-robin, one file system at a time.

Assume a “no” response to all questions asked by
fsck except for
“CONTINUE?”, which is assumed to be affirmative. File
systems will not be opened for writing. This is the default for file
systems to be checked that are concurrently mounted writable.

Enter preen mode: fsck will check all
file systems listed in
fstab(5) according to their
pass number, or any special devices listed on the command line, and will
make minor repairs without human intervention. Any major problems will
cause fsck to exit with a non-zero exit
code, so as to alert any invoking program or script that human
intervention is required.